As society becomes increasingly mobile, mobile computing devices are enjoying a tidal wave of popularity and growth. Cell phones, wireless PDAs, wireless laptops and other mobile communication devices are making impressive inroads with mainstream customers. Constraining this growth and limiting customer satisfaction, however, is the lack of a truly adequate high-coverage-area, inexpensive, small, battery-efficient wireless communication system. Cellular data-transmit telephony-based solutions are far from power-efficient, and impose (relative) cost and size burdens that make them unusable. Likewise, other attempts to solve these problems have proved equally unsuitable. For instance, a few entities have attempted to make use of mobile devices that receive information over Frequency Modulated (FM) sub-carriers. FM sub-carriers (e.g., FM sub-carriers using “SCA” or Subsidiary Communications Authorization) utilize[GW1] the available frequencies above FM stereo within the available modulation bandwidth of an FM station. Sub-carriers are typically leased from radio stations, subject to FCC or other national regulation.
Broadcast communications such as FM sub-carrier based transmissions present unique challenges to ensure data integrity. The transmissions are freely available to anyone with an appropriate receiver and so the channel is highly insecure. Typically the communication is strictly one way, so the sender cannot know the state of the recipients. Various cryptographic methodologies have been utilized in an attempt to create a secure environment for the exchange of information. Many of the proposed cryptographic methodologies are robust solutions that require overhead processing that may be unsuitable in many portable device applications.